Storm water runoff is a form of diffuse or non-point source pollution. It is caused by rain washing pollutants, such as particulate matter, organic matter, heavy metals, and organic toxins into receiving natural bodies of water. As a consequence, natural bodies of water that receive storm water also receive pollutants that have harmful environmental effects. The amount of pollution entering into such receiving bodies of water is related to the degree of urbanization in the surrounding area and the nature of the urbanization. Urbanization results in the paving of land with water-impermeable materials, such as concrete, upon which pollutants tend to accumulate over time. Rain falling upon this type of surface washes the pollutants into the storm water runoff system and thence into the receiving water body.
Several hundred years ago storm water runoff was not a significant problem. Rain fell on earth rich in organic matter formed by the accumulation of decayed vegetation. This organic matter absorbed and filtered the water before it made its way into the groundwater, streams and rivers. In addition, the undisturbed soils lost little particulate matter due to erosion, and there was insignificant, if any, amounts of oil and grease, heavy metals or organic toxins to be carried into the receiving wetlands, streams and rivers.
Now, however, due to increasing urbanization, storm water runoff has been identified as a significant source of pollution in receiving water bodies. Storm water differs from waste water in many ways. Unlike wastewater, which flows more or less continuously year around, storm water is intermittent, and usually shows seasonal peaks. Pollutant concentrations in storm water, in addition to being highly dependent on localized factors, are also correlated with rainfall interval spacing. In other words, the longer the period between storms, the greater the potential for high pollutant concentration(s) when a rainfall event occurs. This is due to the continual accretion of pollutants on surfaces between rainfall episodes. Thus, potential damage to receiving water ecosystems is often greatest after a prolonged dry spell, such as occurs during summer periods over much of the west coast of the United States, when the first storms of the fall season wash particularly concentrated accumulations of toxic materials off roadways and other surfaces. These first flush events occur when receiving streams are at low flow and dilution of the pollutants from the storm water is limited. The degree of pollution of the receiving water body is heavily rainfall intensity dependent. Therefore the heaviest pollutant loading at the end of a dry spell may not occur during the first storms, if these storms are not of sufficient strength to flush pollutants off the rainfall receiving surfaces.
Within a particular storm event, there is also what is known as the "first flush" phenomenon. Generally, the first flush occurs during the first half-hour or so, when the rain is flushing the amassed buildup of pollutants that has accumulated during the interval since the preceding storm, and pollution loadings are highest. Even if the storm lasts several hours or more, contamination levels during the remainder of the storm are usually low or even undetectable.
In an effort to address the pollution problems posed by storm water runoff, U.S. Pat. No. 5,322,629 proposes a method and apparatus for treating storm water. In accordance with the invention of the '629 patent, storm water is passed through a bed of high quality leaf compost material that removes pollutants from the runoff prior to its discharge into a water-receiving system. The patent also illustrates and describes embodiments of an apparatus that facilitates the treatment of the water through the bed of compost. The '629 patent recognizes that the compost may compact over time so that the flow of storm water through each square foot of the bed is reduced. This flow rate through the bed, referred to as its "permeability" in the '629 patent but more correctly termed its "infiltration capacity," is an important parameter. If the infiltration capacity of the bed, i.e., the flow rate in gallons per minute through a square foot of the bend, is low, then a large bed is needed to treat a specified rate of water runoff On the other hand, if the infiltration capacity is high, then a smaller bed may be used but there may be insufficient contact time between the water and the compost for removal of pollutants. Moreover, storm water carries with it sediment, such as sand and other particulates, that can cause gradual clogging up of flow channels in a compost bed so that infiltration capacity is reduced.
In order to enhance long-term infiltration capacity, the '629 patent teaches that at least about 15% by dry weight (of the compost) of gravel or crushed rock should be added to and mixed with the compost to form a bed. Nevertheless, despite such gravel addition, it has been found that over a period of time the infiltration capacity of the compost bed decreases so that the bed must ultimately be replaced. Replacement generally requires some manual labor, and is relatively expensive especially since the operation does not generate revenue for the municipality maintaining the sewer. Municipalities would prefer a storm water runoff treatment system that significantly reduces the frequency with which compost beds must be replaced in order to conserve limited financial resources. However, excessively large beds, to allow for decreasing infiltration capacity over time, are also not favored due to their higher initial capital cost. Thus, a desirable replacement medium for the compost beds should have high initial infiltration capacity, and maintain a significant proportion of this infiltration capacity for a long period of time, preferably a period of several years, that constitutes its useful life.